The aim of the article is to explain how to understand a unduly received benefit within the meaning of the provisions of the Act of 12 march 2004 on Social Assistance, especially because the legislator also defined very similar phrases in other legal acts and in each of them is understood differently. In the author's opinion, since the legislator introduced different legal definitions of the same or similar phrases, administrative authorities should take these differences into account in the practice of applying the law. Moreover, this issue must be noticed in the jurisprudence of administrative courts. The article is an attempt to answer the question whether the use of analogies and auxiliary use of other definitions in matters relating to the unduly received benefit is permissible.